Just realised I've been a bit dumb - this is a ground loop, right? The reason I only get a buzz from the laptop is because thats plugged into the same mains circuit as the amp but the iPod/MP3 player are battery powered and not connected to the mains ground.

Correct me if I'm wrong (quite likely) but I need a ground isolator? At the moment I'm using a star ground - all grounding wires meet at the same point on the metal case which is connected to earth in the plug, so the isolator needs to go between the star and the actual earthing pin, correct?

no.
the chassis must be directly connected to the earth wire in the mains cable. This is the Safety Earth. Do not disturb it.

If you decide to add a Disconnecting Network, then fit it between chassis and the main star (Audio) Ground. You can fit a switch across this Disconnecting Network to short it out if it helps with some installations.

Never fit a switch between the chassis Safety Earth and the mains cable. This must be a permanent mechanical fixing.

__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

Sorry if I was unclear - I was trying to say the same as you suggested: an isolation transformer between the star and chassis. Now that I read it back the previous post is kinda garbled.

I'm still not sure on where to put the isolator... There is one PCB for each channel (the amp is effectively 2 mono amplifiers). Each PCB has 7 connections on it in 3 groups... Below is how they are labelled and how they are wired up.

V+ and V- : These go to the power supply (obviously)
GND : This goes to the chassis

IN : This goes to the signal of the 3.5mm plug
GNd : This goes to the ground of the 3.5mm plug

OUT : This goes to the positive wire of the speakers
GND : This goes to the negative wire of the speakers

Is this correct or should the three ground connections be somehow different? Where does the isolator need to go in this configuration?

What you might also be hearing is the switch mode regulators in all those devices - especially if your LM3886 amp does not include a lowpass input filter. The laptop may be particularly bad as onboard sound chips in laptops typically have very poor isolation between the analogue and digital sides.

Try soldering a 220pF capacitor (a good polypropylene is best, but a NPO ceramic will do) between pins 9 and 10 on the LM3886 chip. This might well cure the problem.